1. Field of the Invention
The present invention relates to a protection circuit for power amplifier for preventing the breakdown of a power amplifier caused by a load change or the like in a high frequency power amplifier such as a mobile communications device.
2. Prior Art
As shown in FIG. 4, a prior art electric power amplifying system comprises an input terminal 201, a power amplifier 202, a power amplifier power supply terminal 203, an output terminal 204, a power amplifier supply voltage control terminal 205, a DC power supply terminal 206, and a power amplifier supply voltage control circuit 207.
A signal inputted through the input terminal 201 is amplified by the power amplifier 202, and then outputted through the output terminal 204. On the other hand, electric power to the power amplifier 202 is supplied through the power amplifier power supply terminal 203. At that time, the power amplifier supply voltage control circuit 207 is controlled through the power amplifier supply voltage control terminal 205 so that a DC voltage inputted through the DC power supply terminal 206 is reduced to a specific voltage.
The power amplifier supply voltage control circuit 207 comprises a buffer amplifier for inputting the voltage of the power amplifier supply voltage control terminal 205 to the power amplifier power supply terminal 203. The power amplifier 202 requires a high current, and hence cannot be connected directly to the power amplifier supply voltage control terminal 205. Thus, the power amplifier supply voltage control circuit 207 is provided.
Nevertheless, this prior art electric power amplifying system does not have the function of protecting the power amplifier from breakdown. Thus, there has been a problem that certain power amplifier operating conditions such as a load change can cause breakdown to the power amplifier.
In a method for avoiding this problem, when an overcurrent flows into a power amplifier, the overcurrent is detected so that the load of the power amplifier is changed. As a result, the power amplifier is protected (e.g., JP-A-2000-295055).
Nevertheless, in this method of changing the load of the power amplifier, the configuration of the variable load circuit is difficult. That is, unless the variable load circuit is provided with the characteristics of a low loss in the normal state, the output power of the power amplifier is reduced, and so is the electric power efficiency.